A new study of wild chimpanzees has found that these close human relatives do not routinely experience menopause, rebutting previous studies which had postulated that female chimpanzees reach reproductive senescence at 35 to 40 years of age.
Together with recent data from wild gorillas and orangutans, the new finding published Thursday in the journal Current Biology suggests that human females are rare or even unique among primates in experiencing a lengthy post-reproductive lifespan.
"We find no evidence that menopause is common among wild chimpanzee populations," says lead author Melissa Emery Thompson, a researcher in anthropology at Harvard University. "It's not at all uncommon for individuals in their 40s and 50s -- quite elderly for wild chimpanzees -- to remain reproductively active."
While wild chimpanzees and humans both experience fertility declines starting in the fourth decade of life, most other human organ systems can remain healthy and functional for many years longer, far outstripping the longevity of the reproductive system and giving many women several decades of post-reproductive life.
By contrast, in chimpanzees reproductive declines occur in tandem with overall mortality. A chimpanzee's life expectancy at birth is only 15 years, and just 7 percent of individuals live to age 40. But females who do reach such advanced ages tend to remain fertile to the end, Emery Thompson and her colleagues found, with 47 percent giving birth once after age 40, including 12 percent observed to give birth twice after age 40.
Female chimpanzees only give birth every 6 years to 8 years on average, and they generally begin reproducing at age 13 to 15. This makes the chimpanzee reproductive profile much longer and flatter than that of humans, whose procreation is concentrated from age 25 to 35.
"Fertility in chimpanzees declines at a similar pace to the decline in survival probability, whereas human reproduction nearly ceases at a time when mortality is still very low," the authors write. "This suggests that reproductive senescence in chimpanzees, unlike in humans, is consistent with the somatic aging process."
In other words, human evolution has resulted in an extended life span without complementary extended reproduction.
(Xinhua News Agency December 14, 2007)
